This invention relates to a method and an arrangement for cleaning a vehicle particulate and more particularly to a method and apparatus for in-situ cleaning of a diesel particulate filter using an exhaust stream.
A diesel particulate filter (DPF) removes organic and inorganic particulate matter (PM) from the exhaust gas stream of an engine. The organic particulate is a complex blend of carbon, hydrogen and oxygen, and is a result of incomplete combustion of the diesel fuel in the cylinder. The inorganic portion of the PM has its source in the additives in the lubrication oil or fuel, and material eroded from the engine surfaces. A large part of these inorganic substances consist of metal oxides of sulphur, for example calcium sulphate. This means they will permanently clog the particulate filters in the long run. Under optimal circumstances, the organic PM will fully combust during filter regeneration and thus leave the filter as gaseous CO2 and H2O. The inorganic component, on the other hand, can not be oxidized inside the filter and converted to gaseous components. Instead it is trapped in the filter as various oxides, commonly termed “ash”. To maintain acceptable performance, the ash must be periodically removed from the filter to prevent it from clogging.
Some installations of diesel particulate filters (DPF) are made on engines which have operating temperatures too low to properly regenerate the filter, for instance, by oxidizing the organic PM. In these cases, the filter can become clogged with PM and potentially reduces the performance of the engine. In addition, a filter with a high soot load has a higher chance of permanent damage through uncontrolled regeneration than one with low soot load. In these cases of insufficiently high operating temperature, regular removal of the soot may be required.
Prior art approaches to filter cleaning, as discussed below, are often complex or comprise moving parts, while others are ineffective at removing tightly bound particulate matter. Other processes can lead to high PM emissions during the cleaning process.
The following examples illustrate a number of prior approaches (e.g., devices and methods) and their disadvantages.
I. Cleaning the DPF when Removed from the Engine
A simple way to clean a filter is with a compressed air hose. The hose is directed into the exit face of the filter, thus blowing the soot out of the wall in the reverse direction to which it was initially deposited (i.e., backwashing or back flushing). This method is imprecise, potentially dangerous (compressed air hazards), requires the full attention of an operator, and if improperly performed can lead to emission of PM from the filter end as well as a poorly cleaned filter.
The dirty filter can be heated in at oven to a high temperature in order to effectively remove the carbon-based particles. This requires a significant energy input and does not remove the inorganic ash. After a heating cycle, the cool down period is significant, and the ash must be removed through vacuuming or washing the filter.
Many systems for cleaning industrial devices utilize a combination of liquid flow and ultrasound, which may be effective, but can be comparatively expensive. In addition, the cleaning liquid can damage the catalytic coating or the matting material which secures the catalyst within its metal housing.
Other methods have described cleaning systems which involve backwashing with a suitable “cleaning fluid” until the filter is clean. However, many catalysts and their matting material are sensitive to large amounts of water or solvents. Solvents have the additional disadvantage of requiring disposal. In addition, the flow of cleaning fluid might not be controlled locally (i.e., a single fluid stream flows over the unit), so that some sections of the filter might not be cleaned as well as others.
A problem with such solution is that all garages or service facilities may not have suitable equipment for filter cleaning. In this case the vehicle may need to be fitted with a replacement filter to remain in operation while the filter is being cleaned. This requires a store of relatively expensive filters to be maintained by the garage or haulage company, in order to have access to replacement filters at all times.
II. Cleaning the DPF while Mounted on the Engine
Methods for collecting particulate using several particulate filters with valves to control the flow path are described in a number of patent documents.
For instance, U.S. Pat. No. 5,930,994 shows a combination of valve settings can start the back-flush of one of the filters i.e., the direction of gas flow is reversed and flows to push the soot out of the filter. The reversed air flow can be heated to allow soot to be burned off as the air passes the DPF.
U.S. Pat. No. 5,725,618 discloses a method which ‘backwashes’ a DPF to remove the particulate and ash collected in the filter. The backwashing occurs while the device is on the vehicle, and an impact air valve is used to provide a pressure wave to dislodge the particulate matter. In order to allow the entire DPF to be cleaned, the filter unit is rotated in order to expose a predetermined sector of the filter unit to the air stream supplied by the impact air valve.
The above “back-flush” methods have the disadvantage that the ash from the lubricating oil never leaves the filter system, as back-flushed ash from one element flows into another element, and manual cleaning will still be required. Also the dislodged material must somehow be removed from the exhaust conduit receiving said material.
III. Rotating Arms
A further method of cleaning a DPF involves a device using a rotating electrical heating element. A portion of the exhaust gas bleeds through a rotating arm and flows over the heating element. The combination of low flow rate and high temperature improves the chance of regeneration.
U.S. Pat. No. 5,116,395 discloses a dust collector with on-board programmable cleaning control. A rotating arm with a plurality of nozzles mounted upon it provides the back-flushing flow, thus causing the particulate to be removed from the bag surface and settle into a collection chamber. The control system operates the min and nozzles to produce jets of cleaning fluid above the various bag units. The arm also contains a sensor for determining the dirtiness of each filter element (a pitot tube is suggested). The system described in the patent has several design elements which make it unsuitable for use in diesel particulate filter applications. First, DPFs are much smaller than dust collectors, and the nozzle designs in the above dust collector a specialized for large filters. A typical DPF is between 15 cm and 32 cm in diameter. The dust collector shown in the patent appears have a relatively large diameter. Second, DPFs can have many thousand cells, and thus focusing air on each individual cell is impractical. Other similar designs for dust collectors have the same shortcomings.
It is desirable to provide an improved method and arrangement for cleaning a vehicle particle filter using an existing exhaust stream in order to overcome the above problems.
In the subsequent text, terms such as “longitudinal” and “transverse” are used to denote directions relative to the main direction of movement of the vehicle. Similarly, terms such as “front” and “rear” are used to define the relative position eta component in relation to said direction of movement.
The invention relates, according to an aspect of the present invention, to a method for cleaning a particulate filter in a vehicle provided with an internal combustion engine, which particulate filter is mounted in an operative position inside an exhaust muffler under normal operation of said engine. The method for performing a cleaning process involves the steps of:                disassembling and removing the particulate filter from a first end of the muffler;        reversing reassembling the particulate filter in a cleaning position at said first end of the muffler;        starting and operating the engine by controlling the engine speed according to a predefined cycle until a predetermined condition is fulfilled;        stopping the engine and returning the particulate filter to its operative position inside the exhaust muffler.        
The engine speed is controlled so that the particulate filter is cleaned by a pulsed exhaust flow from the engine. The engine speed is varied between an upper and a lower limit for a set number times during at least one cycle and measuring a value representing said predetermined condition at the end of said at least one cycle. According to a non-limiting example, the engine speed can be varied from a lower limit selected within the range 700 rpm to 1100 rpm, to an upper limit selected within the range 1500 rpm to 1800 rpm. During each cycle, the engine speed is varied between the upper and lower limit a set number of times over a predetermined period of time. According to a non-limiting example, the number can be selected between 60 and 120 and the period of time can be selected between 60 s and 240 s. The total time taken to complete a cleaning process can be in the range of 10-20 minutes. The above values are merely examples as the selection varies depending on the type and size of engine, the size and the degree of clogging of the particle filter and the type of equipment available for performing the cleaning operation, as will be described below.
Prior to the first cycle the engine speed is increased to a stabilized value during a time period sufficient to allow the exhaust temperature in the muffler to stabilize. The stabilized value for engine speed is preferably at least equal to said upper limit, but can be selected higher or lower than said upper limit. A value representing said predetermined condition is then measured, such as the back pressure or the pressure drop across the muffler or the filter unit, and a first cycle is initiated. The first measurement can be compared to a stored value from a previous cleaning process to determine the degree of clogging. If the filter was replaced during previous service, then a reference value for a clean filter can be used.
At the end of said at least one cycle the engine speed is increased to a limit above said upper limit during a set time period and at least one value representing said predetermined condition is measured. Depending on the type value to be measured, such as the pressure at a suitable location in the exhaust conduit, it can be an advantage or a requirement to allow the exhaust temperature to stabilise before taking a reading, in order to get a correct measurement. Said cycle is repeated until said predetermined condition is fulfilled, or until a maximum number of cycles have been performed. The value representing said predetermined condition can be the exhaust back pressure or the pressure drop across the particulate filter. In most modern engines, exhaust pressure sensors are provided for controlling various parameters in the engine during normal operation. If required, one or more pressure sensors can be provided in the muffler or the particular filter, if standard sensors are not available.
According to one example, the method can be performed by controlling the engine speed during the cleaning process using an electronic control unit. The electronic control unit can be an engine control unit or an external electronic control unit. The electronic control unit can be pre-programmed, whereby the software required for running a cleaning programme is stored on a non-volatile memory or a hard drive in the electronic control unit. Alternatively, the necessary software can be stored on a portable, hand-held unit or on a non-volatile storage unit such as a USB stick or a flash memory that can be connected to the electronic control unit or the engine control unit. In this context, non-volatile data storage is defined to include electrically addressed systems, such as read-only memory (ROM), and mechanically addressed systems, such as hard disks, optical discs, magnetic tapes, holographic memories, etc.
Measurements of said value from a previous cleaning process can be stored in said electronic control unit and be used to set the predetermined condition. In this way, it is possible to compare current readings for said value with values from the previous times the filter was cleaned, or from the time the filler was last replaced. In this way the condition and current state of the particulate filter can be monitored and an expected lifetime can be calculated.
It is also possible to store measurements of at least one value relating to the operation of the engine since the previous cleaning process in said electronic control unit to set the predetermined condition. According to one example, the measured pressure drop across the particulate filter at the end of the previous cleaning process can be set as target for the predetermined condition. Additional factors that can be taken into account when setting the predetermined condition can be the driven distance, engine running time or the fuel and/or oil consumption since the previous cleaning process.
If it is determined that an particulate cleaning process is required under conditions where no electronic control means is available for performing said process, a simplified, emergency cleaning process can be performed. According to this example the engine speed is varied between an upper and a lower limit manually, while measuring the time from the start of the first cycle. The pulsed flow is achieved by pushing the throttle at a predetermined frequency, while monitoring the tachometer, or rpm gauge, to check the upper and lower rpm limits, respectively. This operation can be performed for a predetermined number of cycles or for a period of time estimated to result in a sufficient cleaning of the filter. The particulate filter can then be reversed back into its operative position and the vehicle is again operational.
The invention also relates to an arrangement comprising a vehicle muffler and a particulate filter mounted in an operative position inside said exhaust muffler under normal operation of an engine connected to said muffler. The particulate filter has a first contact surface facing a first end of the muffler, which first contact surface is in sealing contact with a cooperating surface at a first end of the muffler when the particulate filter is in its operative position. Also, the particulate filter has a second contact surface, which second contact surface is in sealing contact with the cooperating surface at the first end of the muffler when the particulate filter is in a filter cleaning position.
The particulate filter is located inside the muffler in its operational position; and that the particulate filter is reversed and located at least partially outside said first end of the muffler in the filter cleaning position.
The first and second contact surfaces are arranged on opposite sides of a flange around the outer periphery at one end of the particulate filter. Each of the first and second contact surfaces comprises an annular contact surface that is identical and mirrored relative to a plane through the flange at right angles to the central axis of the particulate filter.
An ash collecting device is attached to the end of the particulate filter remote from the muffler, when the particulate filter is in a filter cleaning position 18. The ash collecting device is preferably attached to the annular sealing surface of the particulate filter remote from the muffler, when the particulate filter is in a filter cleaning position. Examples of such ash collecting devices can range from a filter bag attached to the end of the particulate filter, to a hose connector provided with a hose connected to a suction means for assisting the removal of ash. Extracted ash can be removed from the exhaust
gas by means of a suitable gas treating device, such as a water scrubber, a cyclone cleaner, an electrostatic filter or a similar device.
The invention also relates to a computer program comprising program code means for performing all the steps of the method described above when said program is run on a computer. The invention further relates to a computer program product comprising program code means stored on a computer readable medium for performing all steps of the method described above when said program product is run on a computer. Finally, the invention relates to a computer system for implementing a method of cleaning a particulate filter in a vehicle comprising a memory for storing program code means and a processor operable to run said program code means for performing all the steps of the method described above.